A non-myeloablating conditioning regimen has been developed that allows allogeneic marrow engraftment and induction of donor-specific transplantation tolerance in mice. In the project proposed, the investigator and her colleagues will focus on the mechanisms of tolerance induction in this model and will use this knowledge to reduce even further the potential toxicity of the pre-BMT conditioning. Aim 1: They will define the mechanism by which residual peripheral CD4+ T cells that escape depletion by mAb treatment are rendered tolerant in mixed chimeras prepared with the non-myeloablative regimen. Aim 2: They will use TCR transgenic and Vb markers to evaluate the mechanism by which additional mAb treatments inactivate residual thymocytes in mice not receiving thymic irradiation. They will evaluate the role of TCR and coreceptor down-modulation and of donor antigen in inactivating residual thymocytes and peripheral T cells that are rendered incapable of rejecting donor marrow. The goal of Aim 3 will be to evaluate recognition of donor and host hematopoietic cells among NK cells derived from both sources in mixed allogeneic chimeras. These studies should allow determination of the elements which dictate self/non-self discrimination among developing NK cells. Important information will be obtained on the factors determining NK cell differentiation, and on the potential obstacle that might be presented by NK cells to the development of lasting mixed chimerism. Aim 4: They will use the information obtained from the first aims to minimize the amount of pre-BMT conditioning required to achieve lasting mixed chimerism and tolerance. They will attempt to diminish the duration of T cell depletion by replacing repeat injections of depleting mAbs with non-depleting reagents. Based upon preliminary studies indicating that engraftment of syngeneic marrow can be achieved without whole body irradiation if high marrow doses are given, they will develop strategies for achieving marrow engraftment without myelosuppression.